Receiver selectivity: In most superheterodyne receivers, which block primarily determines the overall selectivity (i.e., adjacent-channel rejection and bandwidth)?

Introduction / Context:
Selectivity defines how well a receiver separates the desired channel from adjacent channels. In superheterodyne architectures, frequency conversion to a fixed IF allows the use of stable, high-Q filters to set the bandwidth precisely.

Given Data / Assumptions:

• Receiver uses a fixed intermediate frequency (e.g., 455 kHz AM or 10.7 MHz FM).
• IF filters (LC, ceramic, crystal, SAW) provide the bulk of the shaping.

Concept / Approach:

Moving selectivity to a constant frequency means the filter characteristics (bandwidth, skirt selectivity, ripple) are consistent across the entire tuning range. Hence, the IF section predominantly sets receiver selectivity.

Step-by-Step Solution:

Verification / Alternative check:

Service alignment procedures focus on IF filter alignment to correct bandwidth and skirt selectivity, confirming their central role.

Why Other Options Are Wrong:

• Sensitivity relates to minimum discernible signal, not bandwidth.
• Antenna pattern can help but is not the primary selectivity mechanism in superhets.
• Audio and LO power do not define RF/IF bandwidth.

Common Pitfalls:

• Equating sensitivity (gain/noise) with selectivity (filtering).

Final Answer:

Characteristics of the IF section (IF filters and IF amplifiers)